Polyacetal resin is used in extremely diverse fields an an engineering plastic due to its excellent physical properties, such as mechanical, electrical and chemical properties (e.g. chemical and heat resistances). However, new end-use applications require further property improvements and/or enhancements for polyacetal resins.
For example, molded articles to be used as automobile components are expected to exhibit high impact resistance over a wide temperature range. In this connection, the addition of a thermoplastic polyurethane as an impact modifier has been proposed as a means for improving the impact resistance of a polyacetal resin (see, Japanese Patent Laid-Open Nos. 145243/1984 and 19652/1986). However, these prior proposals are disadvantageous due to the poor interfacial adhesion between the polyacetal resin and the thermoplastic polyurethane.
As a result, the functional effect of a polyurethane resin as an impact modifier cannot sufficiently be exhibited. In addition, the obtained molded article exhibits very low weld strength and may have surface delamination problems due to the incompatibility (poor interfacial adhesion) of the polyacetal and polyurethane resins. Delamination of the surface layers and/or decomposition of the polyacetal or polyurethane resins sometimes cause deposits to be formed on mold surfaces. These mold deposits will, in turn, cause dimensional distortion of the molded article and/or will lower the efficiency of the molding operation. Thus, even though the prior proposals to add polyurethane resin to polyacetal base resins does sometimes result in acceptable impact resistance properties being achieved, further improvements have been needed.
Broadly, the present invention resides in blending an impact-resistant effective amount of a polyurethane resin with a polyacetal base resin, wherein the polyacetal resin itself is a mixture of low and high hydroxyl-containing (LHC and HHC, respectively,) polyacetal resins. These LHC and HHC polyacetal resins thus are effective in improving the compatibility of a thermoplastic polyurethane with the polyacetal base resin generally. The resulting polyacetal resin compositions are moreover, free from the disadvantages noted above and exhibit excellent impact resistance properties by blending a thermoplastic polyurethane with a polyacetal base resin component comprised of a low hydroxyl-containing polyacetal resin and high hydroxyl-containing polyacetal resin.
More specifically, the present invention is preferably embodied in polyacetal resin compositions comprising a blend of (A) between about 99 to 40% by weight of a polyacetal base resin component and (B) between about 1 to 60% by weight of a thermoplastic polyurethane, characterized in that the polyacetal base resin component (A) is a mixture which includes between about 99.5 to 1 parts by weight of (A.sub.1) a low hydroxyl-containing polyacetal resin which contains less than 50 mmol/kg hydroxyl groups in its molecule and (A.sub.2) between about 0.5 to 99 parts by weight of a high hydroxyl-containing polyacetal resin containing greater than 50 mmol/kg hydroxyl groups in its molecule.
Further aspects and advantages of this invention will become more clear after careful consideration is given to the detailed description of the preferred exemplary embodiments thereof which follows.